1. Field of the Invention
The present invention relates to a large capacity optical asynchronous transfer mode (ATM) switch, and more particularly to a large capacity optical ATM switch in which the limited capacity of the switch is recovered and large capacity switching can be achieved.
2. Background of Related Art
As for the optical exchange methods, space division multiplexing (SDM), time division multiplexing (TDM), wavelength division multiplexing (WDM) and free-space method have been suggested, each having their advantages and disadvantages. As for the optical ATM in which electric routing is not performed, star coupler type cell routing method is first suggested in Bellcore, and other most ATMs are controlled electrically while high-speed optical bandwidth switching is controlled by optical techniques. Typical examples of such a system are the optical packet exchange system, suggested by NEC, the ultra fast photonic ATM system of NTT, and the space division switch of Hitachi. Recently, frequency routing type time division interconnection network FRONTIERNET of NTT has been disclosed in the academic field of the switch. In the ULPHA switching system (Journal of Lightwave Technology, Vol 10, No. 2, February 1992), as an example, a time division switch system includes n input/output channels 1 as shown in FIG. 1. Electric or optical signals are transmitted to each channel. Each channel is inputted to an input interface module 2. Cells are recovered to remove phase jitter and inserted into time slots which are synchronized by system clocks. Simultaneously, a laser diode 3 which is accommodated in the system is synchronized with the system clock to make ultra high-speed optical signals split to respective cell coders 4 by a coupler 5.
The cells which are inputted to the cell coders 4 are divided into an address part and a data part to be modulated into different wavelengths respectively. At this time, if pulse rows of the laser diode 3 which is synchronized to the master clock of the system are inputted to an external modulator, each channel data are changed into electric signals to modulate the external modulator. In order to prevent cell collision due to the same wavelength which may possibly occur between cells during the exchange, the pulse rows which are outputted from the modulator are compressed by a rate T/n which is obtained by dividing a cell period T with a whole channel number n.
The respective compressed cells are subject to different fiber delays and cells which are inputted to a star coupler 6 become time multiplexed. The cells, which are inputted to the star coupler 6, distributes power to their output terminals, and the address of the compressed cells are detected respectively in the output terminals, so that an optical selector 7 is operated to output only desired cells to the next terminal stage. If the cells of all input ports are to be transmitted to one output terminal, since the total channel number n of compressed cells resides in the original cell period T, these compressed cells are decompressed into an original transmission signal for reception, thereby causing cell collision. For this reason, cell buffers 8 are provided for outputting only one compressed cell to a next terminal per a period T. Cell coders 9 decompress the compressed cells into the original transmission signals in order to transmit them into next optical links or a final receiver after converting them into electric signals. That is, the ULPHA switching system distributes optical power to all output terminals by using the star coupler 6. Therefore, when the system is expanded to increase processing capacity, the optical power which is received by respective receiver terminals is decreased relative to the number of links. Further, if the system is expanded in the time division configuration, the compression required in the cell coder 4 of respective links are also proportionally increased, so that large capacity can not be achieved.
Furthermore, the multi-terminal switching system (Journal of lightwave technology, Vol. 15, No. 3, March 1997) of the FRONTIERNET which has been suggested by NTT, as shown in FIG. 2, includes M.times.N frequency converters 10, N+M frequency routers FR 11, M.times.N frequency switches 12, and K.times.M frequency division multiplexing FDM output buffers 13. Respective input and output terminal links 14 are to be either a transmission line which is frequency division multiplexed or a transmission line which is time division multiplexed, wherein the present invention corresponds to the latter. It is assumed that frequencies of the cell header and payload are different. For example, in case of a channel of the input link, ATM cell which is inputted to the frequency converter 10 is separated into a data part and a header part by a demultiplexer 15 and converted into electric signals of frequency assignment signals by an optical/electron converter 16. A frequency for routing is selected from header information of the electric signal, and a continuous beam of a tunable wavelength converter 17, which is oscillated by a selected frequency, is inputted to an external modulator 18 to be modulated by the first input data signal. Frequency converted cell for routing is inputted to a frequency switch module 19 of destination link by the frequency router 11. In the multi-terminal switching system, when the system is expanded to increase processing capacity, a frequency required for routing at one input link has twice the number of input channels and output links. If all channels of a cell are inputted by the frequency switch 12 of one output link, the cell is selected by k frequency selectors 20 and distributed by a splitter 21. The frequency selector 20 selects only one desired frequency and the selected frequency is converted by a frequency converter 22 to be routed to a final destination. In the worst case where all the channels of the input terminals goes to one destination, the bottle-neck phenomenon occurs. As a result, a large capacity frequency division multiplexing output buffer 13 is required to prevent the bottle-neck phenomenon.
As described hereinabove, in order to realize a large capacity optical switching system, numerous hardwares are required and accordingly variable range of the optical frequency for routing needs to be expanded. However, in view of plane gain bandwidth, the frequency interval is to be finely divided, so that it requires high leveled techniques for maintaining high frequency stability, wherein the manufacturing cost increases.
Furthermore, both switching systems have disadvantages that high leveled techniques are required, and high hardware requirement for increasing the processing capacity, which, accordingly, increases the manufacturing cost.